1. Field of the Invention
The present invention relates to differential gears cast from ductile iron, and more particularly to such a gear cast to near net shape in a flaskless moulding machine from iron inoculated in the mould with various alloying components, heat treated, and austempered, said differential gears having a microstructure predominantly acicular ferrite and high-carbon stable austenite, and characterized by high tensile strength, yield strength and hardness.
2. Description of the Relevant Prior Art
Differential gears are used in both rear wheel drive vehicles and four wheel drive vehicles, and permit the wheels of the vehicle to rotate at different rates while the vehicle is turning. Differential gears do not mesh 100% of the time, but engage only when the vehicle is turning. Because it does not engage all of the time, a differential gear does not have to operate as quietly as a constantly meshing gear. Hence, differential gears, typically made from carburized steel, may be used in an as-forged condition, and do not require additional machining after forging. They do, however, have to be able to withstand very low temperatures and high stress because of load fluxuation encountered by the vehicle, for example, going up or down a hill with a heavy load.
However, forged steel gears are expensive. First, a suitably sized stock piece must be formed of steel. Then, the steel is heated to its forging temperature, then forced into a die. Not only is the steel stock from which the gears are made relatively expensive, but the forging process itself is expensive, uses large amounts of energy, and is slow.
Austempered ductile iron is an alloyed and heat treated ductile cast iron. By varying the composition of the melt, and the heat treating and tempering parameters, austempered ductile iron can display a range of diverse mechanical properties. Characteristically, austempered ductile iron exhibits good yield strength, good impact strength, good tensile strength, high hardness, and outstanding wear resistance. Examples of various austempered ductile iron materials are disclosed in U.S. Pat. Nos. 3,549,430; 3,549,431; 3,860,457; 4,222,793; 4,541,878; 4,737,199; and 4,880,477. However, with conventional casting techniques, intricate castings for differential gears are often defective due to cracks and fractures, etc. because the alloys are inhomogeneous, and contain oxidized alloying elements. Thus, undesirably high local concentrations of some alloying elements and oxides may be present in the casting which create weaknesses. When the intricate castings are subjected to austempering, which involves a quench operation, they are prone to fracture at these weak points. Hence, up until now, austempered, ductile iron has not been widely employed for differential gears and other intricate castings requiring qualities of good strength and wear resistance despite the fact it is otherwise well suited for this purpose.
It is known to employ austempered ductile iron in fabricating gears, but the gears are not cast from the material. For example, Vourinen et al. U.S. Pat. No. 3,860,457 discloses a ductile iron useful as a raw material for fabricating gears. A blank of ductile iron of a particular composition is austenized at a temperature of 900.degree. C. for 2 hours, then quenched in a salt bath at a temperature of 370.degree. C. for a period of time ranging from 10 minutes to 4 hours. This results in an isothermally bainitized blank which is then hardened by work hardening or by machining. Thus, a gear may be machined from such a blank. In U.S. Pat. No. 4,222,793, nodular iron gears are made by casting a nodular iron blank, annealing the blank, ferritizing its microstructure, machining teeth into the blank, austenizing the machined gear, quenching, and shot-peening the surface of the gear. The result is a gear having a surface with high residual compressive strength.
Such austempered ductile iron gears as those disclosed in the two patents referenced in the preceding paragraph are an alternative to conventional carburized, forged steel gears. However, the teeth of these prior art austenized ductile iron gears must be machined from the cast, heat treated metal. Obviously, such a machining step is expensive and difficult. Clearly, it would be advantageous to employ a method whereby differential gears could be cast from nodular iron to near net shape and subsequently austempered, with a minimum of machining.